The Origin of the Erect Cells in the Phloem of the Abietineae

Abstract

1. The occurring on the margins of medullary rays in the phloem of most genera of Abietineae do not exist in the young ray, which consists only of ordinary parenchyma. 2. In young roots of Pinus the phloem shows certain ceils which are essentially short sieve tubes possessing nuclei, occurring in groups in the radial plane. On the xylem side these may merge into one or more narrow rays consisting of tracheids, owing to a diminution and localization of cambial activity. 3. When such a radial group occurs in vertical contact with a medullary ray, cambial activity is sooner or later localized at the edge of the ray, resulting in the production of a border to the ray, such border consisting of sieve cells, which are the erect cells found in mature phloem. 4, Variations of this mode of origin of erect cells occur, such as the cutting of a cell from the end of a sieve tube when it meets the edge of a ray. 5. In young roots and stems marginal ceils may make their appearance in the phloem earlier th...